The field of the invention relates to methods for making electret fibers, and particularly for making electret filter mats for enhancing the removal of particulate matter from gas streams.
Electrostatic forces have been used for many years to enhance the particulate collection efficiency of fibrous filter mats by using a strong electric field oriented in the direction of air flow through a filter structure. A strong electric field can be established, in the direction of air flow through a filter structure by using conductive filaments that are connected to an external high voltage power supply.
One method for creating an electric field in the inner structure of a fibrous filter mat is to coat the fibers with particles of zinc colophony resin. The resin is initially dissolved in a solvent and the fibrous mat is saturated with the solvent plus resin. After evaporation of the solvent, the fibrous structure is mechanically needled to fracture the zinc resin crystals. The frictional effect of particle to particle attrition and/or crystal fracture along internal planes is sufficient to cause the particles to acquire a saturated positive or negative charge. A strong electric field is formed about each particle and the filter structure can remain charged for years at low humidities. Retention will be for a long period of time because the colophony resin is an excellent insulator, having an electrical resistance of 10.sup.21 ohms/square. Another reason for longevity is that the concentration of charged ions or molecules in the atmosphere is relatively low and not adequate for particle neutralization. However, at relative humidities of 50% or greater, the resin will absorb moisture, the material will become conductive, and the field effect will be negated.
An electric field can also be produced in a filter structure when permanent electrostatic charges exist in polymeric fibers that constitute the filter. Such bound permanent electrostatic charge are called "electrets". In a paper by Petryanov et al entitled "FP Fibrous Filtering Materials," Znaniye Press, Moscow (1968), data are given on the enhancement of aerosol collection by electrostatically charged polymeric fibrous filter mats. The authors cautioned that at high humidities, the electret enhancement effect was nonexistent due to charge dissipation. No description is given of the means to impart a permanent electrostatic charge to the fibrous filter.
On the other hand, following the teachings of U.S. Pat. No. 3,998,916, one can make an electret fibrous filter by initial corona charging of a thin polypropylene film, fibrillating the film and using the resulting fibers to construct a nonwoven felted structure. The teachings of this patent are examples of the prior art as it pertains to making a fibrous filter mat starting with a polymeric film structure. However, no technique or technology existed for making electret filter mats starting with an existing nonwoven needled felt.